1. Field of the Invention
The subject invention relates to a viscoelastic polyurethane foam having a density of greater than two and a half pounds per cubic foot. More specifically, the viscoelastic polyurethane foam is flame retardant yet is formed of a composition that is substantially free of flame retardant.
2. Description of the Related Art
Various related art viscoelastic foams are a reaction product of an isocyanate component and an isocyanate-reactive blend. The blends typically include a first isocyanate-reactive component and a second isocyanate-reactive component. Examples of these related art foams are illustrated in U.S. Pat. No. 6,204,300; European Patent Application No. 1,178,061; and PCT Publication WO 01/32736.
Viscoelastic polyurethane foam is currently a niche application in the United States. It is used mainly in home and office furnishings, although a considerable amount of work has been conducted for automotive applications. The market for viscoelastic foam in home furnishings applications is currently estimated at about 25 million lbs/yr. in the United States. While the market size is now relatively small, it is growing at an estimated rate of about 20% to 30% per year.
Viscoelastic foam exhibits slow recovery, and thus high hysteresis, during a compression cycle. They also typically have low ball rebound values. These properties may result from either low airflow, as the recovery is limited by the rate of air re-entering the foam, or by the inherent properties of the foamed polymer. Polymer viscoelasticity is usually temperature-sensitive, and is maximized when the polymer undergoes a glass transition. For the viscoelastic foams currently studied, this glass transition results from vitrification of the polyether soft segment phase. By manipulating the structure and composition of the soft segment phase so that the glass transition temperature approximately coincides with the use temperature of the material, the viscoelastic nature of the material is maximized. When this material is used in a mattress or as a seat cushion, body heat from the user warms a portion of the material, thus softening it. The result is that the cushion molds to the shape of the body part in contact with it, creating a more uniform pressure distribution, which increases comfort. In addition, the remainder of the material remains hard, providing support. Thus, the temperature sensitivity increases the effective support factor of the material, allowing the construction of a mattress without metal springs.
The type of isocyanate component and the functionality and hydroxyl value of isocyanate-reactive component can be selected and formulated such that the glass transition occurs at a temperature at which the foam is used. Viscoelastic foams can also result from low airflow, or porosity. While most of the physical properties of viscoelastic foams resemble those of conventional foams, the resilience of viscoelastic foams is much lower, generally less than about 15%. Suitable applications for viscoelastic foam take advantage of its shape conforming, energy attenuating, and sound damping characteristics. One way to achieve these characteristics is to modify the amount and type of isocyanate-reactive components, isocyanate components, surfactants, catalysts, fillers as in U.S. Pat. No. 4,367,259, or other components, to arrive at foams having low resilience, good softness, and the right processing characteristics. Too often, however, the window for processing these formulations is undesirably narrow.
It is another aspect of these related art patents to produce flexible foam that is flame retardant. Typically, this has been accomplished by adding flame retardant to the compositions, as shown in U.S. Pat. No. 5,420,170. The '170 patent discloses a viscoelastic foam formed from an isocyanate-reactive blend. However, the '170 patent discloses adding flame retardant to make the resulting foam flame resistant. The flame retardant may compromise the integrity of the viscoelasticity, generates toxic chemicals if burned, and adds additional costs to production. However, others have attempted to produce the flame-retardant foam without employing any flame retardant in the composition. These approaches modify the amount of the isocyanate-reactive blend as shown in U.S. Pat. No. 6,495,611. The '611 patent discloses an isocyanate-reactive blend formed from hydrophilic compounds and hydrophobic compounds. The '611 patent also discloses utilizing flame retardant to increase the flame resistance, but it does not disclose the foam formed from the isocyanate-reactive blend as having any viscoelastic properties. Other related art foams are shown in U.S. Pat. Nos. 4,334,031; 4,374,935; and 4,568,702; PCT Publication WO 01/25305; European Patent No. 0934962; and European Patent Application Nos. 1125958 and 0778301. However, none of these related art patents discloses or suggests the unique and novel viscoelastic polyurethane foam of the subject invention.
Other approaches to making viscoelastic foam hinge on finding the right mixture of polyether polyols and other components. For example, U.S. Pat. No. 4,987,156 arrives at a soft, low-resilience foam with a mixture of high and low molecular weight polyols, each of which has a hydroxyl functionality of at least 2, and a plasticizer having a solidification point less than −20 degrees C. However, the '156 patent does not disclose a viscoelastic foam. U.S. Pat. No. 5,420,170 teaches use of a mixture that includes one polyol having a hydroxyl functionality of 2.3–2.8, and another polyol having functionality 2–3. U.S. Pat. No. 5,919,395 takes a similar approach with a polyol mixture that contains a 2500 to 6500 molecular weight polyol having a functionality of 2.5 to 6 and a rigid polyol having molecular weight 300 to 1000 and a functionality of 2.5 to 6. Neither the '170 patent nor the '395 patent disclose adding a chain extender to the composition to modify the glass transition temperature of the foams.
Another related art composition is disclosed in a paper titled “Novel MDI-Based Slabstock Foam Technology” by Lutter and Mente. The composition disclosed produces a viscoelastic foam from a isocyanate prepolymer, a flexible polyol, and an ethylene-oxide rich polyol. However, the paper does not disclose a chain extender present in significant amounts to produce the viscoelastic foam having the improved properties.
Monols, such as monofunctional alcohols, have also been included in flexible polyurethane foams for various reasons, but they have rarely been used in a viscoelastic foam such as U.S. Pat. No. 6,391,935. The '935 patent discloses a TDI based viscoelastic foam and it does not disclose a foam substantially free of TDI. The '935 patent also does not disclose using a chain extender to modify the glass transition temperature of the foam. Most references that include a monol teach compositions that form foams having high resilience, such as, U.S. Pat. Nos. 4,981,880; 3,875,086; and 3,405,077. However, none of these references disclose using a composition being substantially free of flame retardant and that includes chain extenders to produce the viscoelastic foam. Other references teach the use of low molecular weight monofunctional materials. For example, U.S. Pat. No. 5,631,319 teaches use of a C1–C25 monoalcohol combined with a hydroxyketone in non-viscoelastic foam. U.S. Pat. No. 4,209,593 discloses a naphthol or other “bulky” monohydroxy compound to make an energy-absorbing foam. Both the '319 patent and the '593 do not disclose a viscoelastic foam according to the subject invention. Including low-molecular-weight (<1000), high hydroxyl number (>60 mg KOH/g) monols in viscoelastic foams adversely impacts important foam properties, particularly compression sets. In addition, any monol can remain largely unreacted, especially in a low-index formulation, resulting in a foam that is oily to the touch and provide poor “hand feel”.
European Patent Application No. 0913414 teaches viscoelastic polyurethane foams that may contain a polyether monol. The monol, which has a molecular weight less than 1500, is used with a polyol that has a molecular weight greater than 1800. All of the examples produce a foam formed with a low isocyanate index of less than 90. U.S. Pat. No. 4,950,695 teaches use of a monofunctional alcohol or polyether to soften flexible polyurethane foams. The formulations also include a 2000 to 6500 molecular weight triol. The '695 patent does not disclose a viscoelastic foam that is flame retardant without additional flame retardant being added.
These foams are characterized by one or more inadequacies. Accordingly, it would be advantageous to provide a viscoelastic polyurethane foam that overcomes these inadequacies. Moreover, it would be advantageous to provide a viscoelastic foam that has a density from greater than two and a half pounds per cubic foot formed from a composition that is substantially free of flame retardant and that is a reaction product of an isocyanate component and an isocyanate-reactive blend, such that the foam is flame retardant.